Water · Worked step by step
How many gallons are in a 30 foot round tank?
If math has never been your thing, you are in the right place. Try it on paper first, then we write it out with you, one small step at a time, and explain every step in plain words so you never lose the thread.
The problem
A round tank is 30 feet across and holds water 32 feet deep. How many gallons does it hold?
Grab a pen and paper first
Give it a real try on paper before you watch. If you get stuck, take a hint. If you go blank, that is fine, press play and we will walk it through together.
Watch it solved, with you
Every step is written out slowly and explained in plain words, so you always know what just happened. Nobody skips ahead and leaves you behind.
Press play and I’ll write it out with you, one small step at a time.
Every step gets a plain-words explanation here, so you always know exactly what just happened and why.
The written version, step by step
- 1
Write down what you know
across = 30 ft • depth = 32 ft
- The tank is 30 feet across (that is the diameter) and 32 feet of water deep.
- Round tanks scare people, but this is the same idea as a box with one extra number.
- Write both down and let's go.
Quick trick: "Across" means diameter. If a problem gives the radius instead, double it to get across.
- 2
Find the circle floor with 0.785
area = 0.785 × 30 × 30 • area = 706.5 sq ft
- The floor is a circle. Its area is 0.785 × across × across.
- First 30 × 30 = 900.
- Then 900 × 0.785 = 706.5 square feet. That is the floor.
Quick trick: 0.785 is on your formula sheet too. Just remember: round floor = 0.785 × across × across.
- 3
Stack the floor up by the depth
cubic feet = 706.5 × 32 • cubic feet = 22,608
- Now take that floor and stack it 32 feet deep.
- 706.5 × 32 = 22,608 cubic feet of space.
- We are one step from gallons.
- 4
Turn cubic feet into gallons
gallons = 22,608 × 7.48 • gallons = 169,108
- Last step, same as always: × 7.48 for gallons.
- 22,608 × 7.48 is about 169,108 gallons.
- That is your answer.
Quick trick: Big number, but the steps never change: floor area, then cubic feet, then gallons.
Remember this for good
For a round tank: 0.785 × across × across gives the circle floor area. Then × depth = cubic feet, then × 7.48 = gallons.
A round tank is just a box with the corners shaved off. 0.785 is the "corner-shaving" number (a circle is about 78.5% of the square around it). Use it, and a round tank is as easy as a box.
Solve it fast next time
- Round tank = same as a box, but start with 0.785 × across × across for the floor.
- "Across" is the diameter. Don't cut it in half here (halving gives radius, a different formula).
- Same finish every time: × depth for cubic feet, × 7.48 for gallons.
The answer: About 169,108 gallons.